1. Field of the Invention
The present invention relates to a cellular radio communication system and, in particular, to a method and apparatus for controlling radio link based on whether radio link failure is detected.
2. Description of the Related Art
Recently, many researches are being conducted on the Orthogonal Frequency Division Multiple Access (OFDMA) and Single Carrier Frequency Division Multiple Access (SC-FDMA) as useful schemes for high speed data transmission over a radio channel. In such multiple access schemes, the user-specific data and/or control information are mapped to time-frequency resources without overlapped from each other, i.e. maintaining orthogonality, to identify the user-specific data and/or control information.
In a cellular communication system, one of the significant factors to provide high-speed wireless data service is bandwidth scalability for dynamic resource allocation. For example, Long Term Evolution (LTE) system can support the bandwidths of 20/15/10/5/3/1.4 MHz. The carriers can provide services with at least one of the bandwidths, and the user equipments can have different capabilities such that some supports only 1.4 MHz bandwidth and others up to 20 MHz bandwidth. The LTE-Advanced (LTE-A) system, aiming at achieving the requirements of the IMT-Advanced service, can provide broadband service by aggregating carries up to 100 MHz.
The LTE-A system needs the bandwidth wider than that of LTE system for high-speed data transmission. Simultaneously, the LTE-A system needs to be backward compatible with the LTE system such that the LTE UEs can access the services of the LTE-Advanced system. For this purpose, the entire system bandwidth of the LTE-A system is divided into sub-bands or component carriers that have a bandwidth supporting transmission or reception of the LTE UE and can be aggregated for supporting the high speed data transmission of the LTE-A system in the transmission/reception process of the legacy LTE system per component carrier.
Meanwhile, if the radio rank quality between a transmitter and a receiver is degraded below a predetermined level, it is difficult to expect the normal data communication. Accordingly, the UE or the eNB monitors the radio rank quality to determine the Radio Link Failure (hereinafter, referred to as RLF) and performs the operation corresponding to the RLF. A description is made of a procedure for a UE to determine whether RLF occurs in the legacy system not supporting carrier aggregation with reference to FIG. 1.
The UE connects to an eNB to transmit and receive data (108), recognizes, if the radio link quality with the base station degrades continuously for a predetermined observation period, the problem of the radio link (110), and determines, if the radio link quality does not recovered for a predetermined time T1 (112), occurrence of RLF (104). If the RLF is detected, the UE attempts to connect to a cell having the best radio link quality among the neighbor cells for a predetermined time duration T2 (114) to continue, if the attempt is successful, data transmission/reception and end, if the attempt fails, end all the transmission/reception operations and transition to idle mode (116).
The procedure related to the RLF is defined for the conventional system without consideration of carrier aggregation and thus there is a need to define a procedure for controlling the radio link depending on whether RLF occurs or not in the wireless communication system supporting carrier aggregation.